Unicast Routing
20-20
20
NSSA
– A not-so-stubby area (NSSA) can be configured to control the use of default
routes for Area Border Routers (ABRs) and Autonomous System Boundary Routers
(ASBRs), or external routes learned from other routing domains and imported
through an ABR.
An NSSA is similar to a stub. It blocks most external routing information, and can be
configured to advertise a single default route for traffic passing between the NSSA
and other areas within the autonomous system (AS) when the router is an ABR.
An NSSA can also import external routes from one or more small routing domains
that are not part of the AS, such as a RIP domain or locally configured static routes.
This external AS routing information is generated by the NSSA’s ASBR and
advertised only within the NSSA. By default, these routes are not flooded onto the
backbone or into any other area by ABRs. However, the NSSA’s ABRs will convert
NSSA external LSAs (Type 7) into external LSAs (Type-5) which are propagated into
other areas within the AS.
There are no external routes in an OSPF stub area, so routes cannot be
redistributed from another protocol into a stub area. On the other hand, an NSSA
allows external routes from another protocol to be redistributed into its own area,
and then leaked to adjacent areas.
Routes that can be advertised with NSSA external LSAs include network
destinations outside the AS learned via OSPF, the default route, static routes, routes
derived from other routing protocols such as RIP, or directly connected networks that
are not running OSPF.
Also, note that unlike stub areas, all Type-3 summary LSAs are always imported into
NSSAs to ensure that internal routes are always chosen over Type-7 NSSA external
routes.
An NSSA can be used to simplify administration when connecting a central site
using OSPF to a remote site that is using a different routing protocol. OSPF can be
easily extended to cover the remote connection by defining the area between the
central router and the remote router as an NSSA.
Default Cost –
This specifies a cost for the default summary route sent into a stub or
not-so-stubby area (NSSA) from an Area Border Router (ABR).
backbone
NSSA
ABR
default external
route for local AS
ASBR
external network
Router
default external
route for another
routing domain
7
5
AS
Summary of Contents for 8926EM
Page 6: ...ii ...
Page 34: ...Getting Started ...
Page 44: ...Introduction 1 10 1 ...
Page 62: ...Initial Configuration 2 18 2 ...
Page 64: ...Switch Management ...
Page 76: ...Configuring the Switch 3 12 3 ...
Page 118: ...Basic Management Tasks 4 42 4 ...
Page 164: ...User Authentication 6 28 6 ...
Page 176: ...Access Control Lists 7 12 7 ...
Page 284: ...Quality of Service 14 8 14 ...
Page 294: ...Multicast Filtering 15 10 15 ...
Page 300: ...Domain Name Service 16 6 16 ...
Page 310: ...Dynamic Host Configuration Protocol 17 10 17 ...
Page 320: ...Configuring Router Redundancy 18 10 18 ...
Page 344: ...IP Routing 19 24 19 ...
Page 356: ...Unicast Routing 20 12 20 Web Click Routing Protocol RIP Statistics Figure 20 5 RIP Statistics ...
Page 386: ...Unicast Routing 20 42 20 ...
Page 388: ...Command Line Interface ...
Page 400: ...Overview of the Command Line Interface 21 12 21 ...
Page 466: ...SNMP Commands 24 16 24 ...
Page 520: ...Access Control List Commands 26 18 26 ...
Page 546: ...Rate Limit Commands 30 2 30 ...
Page 612: ...VLAN Commands 34 24 34 ...
Page 626: ...Class of Service Commands 35 14 35 ...
Page 670: ...DHCP Commands 39 16 39 ...
Page 716: ...IP Interface Commands 41 36 41 ...
Page 768: ...IP Routing Commands 42 52 42 ...
Page 770: ...Appendices ...
Page 791: ......